With conventional training idlers, the motion of the idler training the belt is generally restricted to a slewing action about a central vertical axis normal to the width of the belt.
The angle between the training roller and the belt generates a lateral re-aligning influence on the belt that has become misaligned from its central position due to other influences.
The idler slewing frame is normally driven by servo rollers on either side of the belt, which in turn are activated by the belt as it misaligns laterally. The re-aligning influence that a training idler of this configuration can exert on the belt is limited by the single degree-of-freedom of training movement available to the idler and the frictional force that can be developed between the belt and the training idler.
French patent specification no. 2586235 discloses a training idler in which the idler roller frame is rotatably mounted on a stub axle fitted to a support frame. Such mounting between the two frames provides only one degree of freedom of movement, namely rotational movement about the stub axle. However, the stub axle is angled at 60.degree. to the horizontal so that any slewing of the idler roller also results in tilting of the idler roller. There is a fixed relationship between the slewing and tilting actions of the idler roller which is determined by the angle of inclination of the axis of rotation of the idler frame.
In the training idler system of French patent specification 2586235, a positive force is applied to the idler frame to cause the frame to rotate, and hence tilt. This force is actuated by an optical sensor which detects misalignment of the belt. However, such an arrangement has several disadvantages. First, it is necessary to provide a mechanism to provide the positive tilting force. Such a mechanism increases the cost of the roller, as well as servicing requirements. Secondly, it is necessary to provide sensors to detect misalignment of the belt. Optical sensors are generally unsuitable for use in the dusty environments in which such belts normally operate, and must be cleaned regularly. Further, a source of power must be provided for each training idler.